MEDIA STUDIES Transmissibility of the Ice Bucket Challenge among globally influential celebrities: retrospective cohort study Michael Y Ni, Brandford H Y Chan, Gabriel M Leung, Eric H Y Lau, Herbert Pang School of Public Health, Li Ka Shing OBJECTIVES To estimate the transmissibility of the Ice The most commonly used metric of transmissibility is Faculty of Medicine, The University Bucket Challenge among globally influential celebrities the basic reproduction number (R0), defined as the num- of Hong Kong, Hong Kong Special and to identify associated risk factors. ber of secondary cases generated by a single index in a Administrative Region, China 7 Correspondence to: M Y Ni DESIGN Retrospective cohort study. fully susceptible population. The value of R0 is a major
[email protected] determinant of the size of an epidemic, and an infection SETTING Social media (YouTube, Facebook, Twitter, Cite this as: BMJ 2014;349:g7185 can only be self sustaining if R is greater than 1. The R Instagram). 0 0 doi: 10.1136/bmj.g7185 also provides a measure of the effort required to control PARTICIPANTS David Beckham, Cristiano Ronaldo, the epidemic.7 8 We estimated the transmissibility of the Benedict Cumberbatch, Stephen Hawking, Mark Ice Bucket Challenge among globally influential celebrities Zuckerberg, Oprah Winfrey, Homer Simpson, and Kermit and identified the associated risk factors. the Frog were defined as index cases. We included contacts up to the fifth generation seeded from each index case and Methods enrolled a total of 99 participants into the cohort. Participants MAIN OUTCOME MEASURES Basic reproduction number We considered globally influential celebrities who had R0, serial interval of accepting the challenge, and odds ratios undertaken the Ice Bucket Challenge to be eligible for inclu- of risk factors based on fully observed nomination chains.